A typical aggregate dryer used in asphalt plants is known as a drum mixer. Such a dryer rotates and contains lifters for showering the aggregate through hot gases given off by combustion of fuel in the burner. A typical drum mixer lifter arrangement includes the longitudinal arrangement of lifters and can be divided into three main types:
(A) Combustion zone lifters, PA1 (B) Drying zone lifters, PA1 (C) Mixing zone lifters.
The combustion zone lifters are lifters designed to produce a veil-free zone to allow combustion to take place. Drying zone lifters allow for showering and veiling of the aggregate across the cross-sectional area of the drum. As the material falls in a downward trajectory, the hot gases from the burner pass around the material to cause heat transfer between hot gases and the relatively cool material. The last zone is the mixing zone and includes lifters that ensure that the asphalt mixed with the aggregate becomes a homogeneous mix for discharge out of the drum. The asphalt in liquid form is injected generally, just at the beginning of the mixing zone lifters.
Exemplifying prior art apparatus is disclosed in U.S. Pat. No. 2,421,345 wherein drum lifters lift aggregate and permit it to drop across the interior of the drum, but no attention or awareness of the heat transfer problem is considered. U.S. Pat. No. 3,025,611 discloses lifters for lifting the aggregates and dropping them through a hot gas stream. The lifters are provided for adequate draft for the hot gas stream to prevent plugging of the dryer. U.S. Pat. No. 3,641,683 discloses a fixed and adjustable lifter arrangement specifically constructed to ensure an adequate draft for the hot gas stream. U.S. Pat. No. 3,940,120 discloses an arrangement of drum lifters (called buckets) which increases the veiling effect of the falling aggregate to reduce particulate emissions.
The purpose of this disclosure, the interaction of the aggregate and hot gases in the drying zone, will be discussed. It is well known that, in a dryer, the longer the drying zone lifter section is, the more complete the heat transfer is between the exhaust gas and the material being dried.
An infinitely long drum would provide complete heat transfer between the gas and the material. However, in a drum mixer having three zones of lifters, the drying zone, by necessity, is relatively short, thus for efficient operation, a high rate of heat transfer is required in this zone.
FIG. 8 shows a typical cross-section through a drum mixer and illustrates the showering effect obtained in a conventional lifter arrangement. It can be seen that, due to this showering action, the separate pieces of material have a chance to come in direct contact with the hot exhaust gas and heat transfer will take place between the gases and the material.
It can be shown that the showering time for the material in a drum mixer or dryer occupies only 5-10% of the total time that the material is in the dryer. For the balance of the time, the material is in the lifter or in the bed of the material sliding down the sloping drum. While in the latter two positions, the heat transfer will take place at a slower rate due to less material being in direct contact with the gases and only surface material receiving radiant heat.
A recent development in drum mixers has been to use these machines to reprocess reclaimed asphalt paving (RAP) material by mixing it with some virgin material and adding the necessary asphalt cement to make a satisfactory new mix. In such a recycle drum mixer arrangement, where reclaimed asphalt paving (RAP) is fed into a chute positioned over the center of the drum, the chute empties into a shroud surrounding the drum, and the material passes through holes which pierce the periphery of the drum into angled chutes which discharge into the drum towards the discharge end. At the same time, virgin material, such as aggregate, is fed into the feed end of the drum. Asphalt cement is added to the mix slightly downstrean of the RAP material entry point. In the recycle operation, the discharged material is made up of a percentage of RAP material injected into the center of the drum plus a percentage of virgin material fed into the feed end of the drum. These percentage ratios can be 40/60, 50/50, 60/40, 70/30, etc. Because a relatively small amount of virgin material is required in some of these mix ratios, the lifters as shown in FIG. 8 provide only a very thin veil. This allows the flame and exhaust gases to penetrate through the veil without sufficient reduction in the exhaust gas heat. Because the veil of material is relatively thin, the hot gases do not contact enough material to give up their heat, thereby causing excessive heating of the RAP material as it enters the drum. The RAP material is usually material milled off of an old road surface and contains many fines consisting of minus 200 micron particles. These fine particles are swept into the gas stream and if the gas has not been sufficiently cooled in passing through the drying zone, then the excessive heat in the gases will cause smoking of the asphalt material coating the RAP fines. It is also possible to cause the virgin asphalt, as its injection point, to scorch and therefore cause smoke emissions if the gas temperature is too high at the asphalt injection point. This can occur in both recycling and virgin material processes. Smoking of the virgin asphalt cement can be caused by a thin veil in the drying zone, incorrectly positioned asphalt injection point, or a low smoke point asphalt cement.